Agriculture Reference
In-Depth Information
More recently, these techniques have been supplemented by the advent of molecular
markers, whereby the precise location of recombination events along chromosomes can
be identifi ed through the existence of DNA polymorphisms in the region of chromo-
some translocations. These molecular tags have been used for example to identify crosses
made with the ph1b mutant that deliver the smallest possible segments of DNA whilst
still retaining the
Sr
26 gene for stem rust resistance in wheat (Dundas and Shepherd,
1998). This has led to breeding lines being developed that should avoid the yield penalty
associated with this otherwise successfully used gene in the past (Dundas, 2007).
As well as the crossing of wild species or ancestors with modern wheats, another
strategy in which there has been much recent interest has been the use of the diverse
diploid and tetraploid ancestors to recreate modern hexaploid wheat. At the International
Maize and Wheat Improvement Centre (CIMMYT) in Mexico, an extensive collection of
'synthetic' wheat lines based on crosses between different
Ae. tauschii
(syn.
Ae. squarrosa
)
accessions, donor of the D genome in wheat, and tetraploid durums
T.
turgidum
ssp
. durum
(AABB genome), have been developed (Mujeeb-Kazi, 1996). Initial crosses were made
fertile by chromosome doubling using colchicine and lines with good agronomic adapta-
tion were identifi ed following hybridisation with elite CIMMYT spring bread wheat lines
(AABBDD). At the same time, similar synthetic wheats, based in this case on winter
wheat crosses, were made in the USA which have led to the identifi cation of bread wheat
lines with useful resistances including to Hessian fl y (Cox
et al.,
1995). Synthetic wheats
have also been developed in Australia and have been used to transfer resistance to cereal
cyst nematode (CCN) caused by
Heterodera avenae
(Eastwood
et al
., 1991) to adapted
wheat lines. Together, these and other projects have released valuable new germplasm
for resistance to a wide diversity of pathogens, as well as other useful traits in an easily
accessible form for breeding programs (reviewed in van Ginkel & Ogbonnaya, 2006).
The fi rst commercial varieties based on the CIMMYT synthetic wheats, Chuanmai42 and
Carmona, were released in 2003 to farmers in China and Spain respectively (van Ginkel
& Ogbonnaya, 2006). These varieties were not selected specifi cally for disease resistance,
but for their high yield and agronomic adaptation. Future varieties are expected to draw
on the impressive range of novel variation for disease resistance.
6.3.3
Transgenic resistance
Genetic manipulation (GM) technologies involving the sequencing and cloning of
resistance genes are making the use of distantly related and alien species a much easier
option. Many resistance genes have now been isolated from a broad range of hosts (Hulbert
et al.,
2001), their DNA sequenced and signifi cant similarities in their structure identifi ed.
Most have a leucine-rich repeat region (LRR) and a nucleotide-binding site (NBS) region.
The LRR regions have been specifi cally linked to resistance (R) gene specifi city, as well
as to defence response signalling in the plant, although at this stage generalisations about
the specifi c functions of these regions are likely to hide a great deal of complexity. Another
important group of proteins associated with resistance responses are kinases. These prob-
ably interact with NBS-LRR proteins in the host resistance pathway. In the case of the
rice bacterial blight resistance gene
Xa
21, a kinase is linked with an LRR region and a
short transmembrane (TM) region that allows the expressed protein to straddle the cell
membrane, with the kinase region located in the cytoplasm (Song
et al.,
1995). In contrast,
